Stabilization of the dipole alignment of poled nonlinear optical

Macromolecules 1993,26, 5303-5309

5303

Stabilization of the Dipole Alignment of Poled Nonlinear Optical Polymers by Ultrastructure Synthesis Chengzeng Xu, Bo Wu, Olga Todorova, and Larry R. Dalton' Department of Chemistry, University of Southern California, Los Angeles, California 90089-1062 Yongqiang Shi, Peter M.Ranon, and William H.Steier Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089-0483 Received March 23, 1993; Revised Manuscript Received June 29, 1993.

ABSTRACT A new approach to attaininglong term and high temperaturestability of second-order nonlinear optical effects has been developed. Two side-chain copolymers containing dipole-end cross-likable chromophore pendants were synthesized from a double-end cross-linkable (DEC) chromophore monomer. The polymer solutions containing a cross-linker were spin cast into optical quality thin fiis, which were subsequently poled by an electric field, using a corona configuration, and thermally cross-linked. Secondorder susceptibilities,x@),on the order of lCk7 esu, of the polymer films were measured by the second harmonic generation (SHG) at 1.06-pm fundamental wavelength. The cross-linked films show no SHG signal decay for thousands of hours at room temperature. Thermal stability studies demonstratethat long term stability of the second-order nonlinear optical effects at 125 O C has been realized using the DEC approach. Introduction Electrooptic device application of second-order nonlinear optical (NLO) materials requires a large optical nonlinearity and long term stability of the nonlinearity in the working temperature range 60-125 "C. After electric field poling, NLO polymers exhibit large optical nonlinearities. Covalently incorporating NLO moieties into polymers as pendants or part of the backbone is effective in improving the nonlinearity and in imparting good processibility to the NLO materials.' Poled polymers, however, exhibit a major disadvantage when compared to inorganic crystals in terms of long term NLO thermostability. In the absence of a poling electric field, the dipole orientation in poled polymers tends to relax to the thermodynamicallymore stable random structure through polymer chain segmental motions and pendant rotations, leading to the decay of the nonlinearity. Because of this, poled polymers are greatly limited for applications in electrooptics as an alternative to crystals, although they are superior in other aspects such as large nonlinearity, excellent p r d b i l i t y and low cost of fabrication. Therefore, stabilizing the poling-induced noncentrosymmetric lattice is a challenging and critical undertaking. In addressingthis problem, cross-linkingreactions have been proven very A number of approaches have been explored to realize stabilization of the polinginduced dipole alignment through utilization of crosslinkable polymers. These include thermosetting prepolymers,u side-chain polymers,7-12 main-chain polymers,'%" andguest-host composites.1620 Other methods of stabilizingthepoling-induced order include doping NLO chromophores into polymer matrices with high glass transition temperatures (T,)and attaching the chromophores covalently onto the backbone of high Tg polymers, such as p o l y i m i d e ~ . ~Al~l-of~these ~ approaches are somewhat successful in improving the temporal stability of the nonlinearity. So far, however, there has not been a polymer material prepared that meets the aforementionedrequirements. Most of the materials still Abstract published in Advance ACS Abstracts, September 1, 1993.

do not possess enough NLO stability, especially at elevated temperatures (e.g., 90-125 "C), and some materials gain the stability at the sacrifice of optical nonlinearity, film quality, or processibility. To realize acceptable NLO stability without sacrificing the chromophore number density (and hence the secondorder optical nonlinearity) and processibility of NLO polymers, we have developed a novel class of double-end cross-linkable (DEC) chromophores.lo Unlike other chromophores with single-functionalityends, the DEC chromophores have two functionalities at each end. More significantly, the functional groups at the two ends of a DEC chormophore are different in functionality. Noncomplementary functionalities at the two ends make it possible for a DEC monomer to undergo two types of polymerization reactions with different mechanisms (Le., condensation and addition polymerization). This enables us to synthesize processible polymers that are crosslinkable at the free end of the chromophore pendant. After electric poling and cross-linking, both ends of a chromophore dipole are locked into the polymer network by strong chemical bonds, which is expected to dramatically restrain the motions of the dipoles and therefore minimize relaxation of the noncentrosymmetric order. Amino-sulfone azobenzene chromophoreswere used in this paper because of their sizable first molecular hyperopolarizabilities(0) and the flexibilityof introducing useful functionalities at the ends of the chromophores. Robello, Williams, and co-workers at Eastman KodakaI25 have pioneered the use of this class of chromophores for nonlinear optics. They have introduced a single functionality at each end of an amino-sulfone azobenzene chromophore, from which they have synthesized several head-to-tail main-chain NLO polymers.a Using a modified procedure, we have synthesized an amino-sulfone chromophore with one hydroxyl group at each end and synthesizeda cross-linkablemain-chainNLO polymer with the chromophores randomly bonded in the polymer ba~kb0ne.l~In this paper, the detailed design and synthesisof a DEC chromophoremonomer with an aminosulfone donor-acceptor pair as well as its dipole-end crosslinkable polymers will be discussed.

0024-9297/93/2226-5303$04.00/00 1993 American Chemical Society

Macromolecules, Vol. 26, No. 20, 1993

5304 Xu et al.

1 19

NR2

Monomer 2 Figum 1. Reaction scheme for the syntheses of monomer 1 and monomer 2.

Experimental Section General Methods. Conventional lH and 13C NMR spectra were taken from a Bruker-250 spectrometer operating at 250 MHz, and two dimensional (2D) NMR spectra were obtained using a Bruker AMX-500 spectrometer operating at 500 MHz. The chemical shifts are referenced to tetramethylsilane (TMS) internal standard. All FTIR spectra were measured with a Perkin-Elmer 1760FTIR spectrophotometer using pressed KBr pellets. Glass transition temperatures (T,)and thermal decomposition temperatures (Td) were determined with Perkin-Elmer DSC-7 and TGA-7 systems, respectively, at a heating rate of 20 OC/min under an argon atmosphere. Elemental analysis was performed by Atlantic Microlab, Inc. Polymer molecular weights were measured by size exclusion chromatography (SEC) using tetrahydrofuran (THF) as the eluent and polystyrene as the standard. Materials. The solventa used in the moisture sensitive reactions were dried and stored in a drybox before each use. 1,2-Dimethoxyethane (DME) and THF were dried by refluxing with sodium and the indicator benzophenone. Pyridine (Py) was dried by distillation over barium oxide. The comonomers, methyl methacrylate (MMA), methacryloyl chloride, and 2-hydroxyethyl methacrylate (HEMA), were purified by distillation. 4-Aminophenyl 6-hydroxyhexyl sulfone (2) was synthesized according to the published literature procedures.u All other starting materials, reagents, and solvents purchased fromvarious chemical companies were analytical reagent grade and were used without further purification unless otherwise noted. The syn-

theses of monomer 1and monomer 2 were carried out according to the reaction scheme shown in Figure 1. N~-Bis(2-((methylthio)methoxy)ethyl)aniline (1). A three-neck flask containing Nfl-bis(2-hydroxyethyl)aniline (9.8 g, 54 mmol) was vacuum dried at 60 OC and back-fied with argon. Dry DME was added into the flask under argon, and the solution was cooled to 0 “C in a sal-ice bath. To the stirred solution was added sodium hydride (5.2 g, 0.22 mol). After the solution stopped bubbling, 18 g (0.11 mol) of potassium iodide and then 10.4 g (0.11 mol) of chloromethyl methyl sulfide (ClMTM) were added. After 1h at 0 OC, the reaction solution was stirred overnight in an unattended ice bath, which gradually reached room temperature. The resulting suspensionwas poured into 250 mL of water, and the mixture separated into three layers. The aqueous layer (middle layer) and the bottom layer were extracted with ether. The ether portion was washed with brine, dried over anhydrous potassium carbonate, and then concentrated. The pale yellow oil obtained was purified by column chromatography (silica gel; hexanes/ether, 1:4 by volume), affording 8.5 g (53%) of oily product. 1H NMR (CDCh): 6 7.21 (t, J = 7.2 Hz, 2H), 6.71 (m,br, 3H), 4.63 (a, 4H), 3.70 (t,J = 6.2 Hz, 4H), 3.60 (t, J = 6.2 Hz, 4H), 2.13 (8, 6H). d(Dimethylamino)-4’-( (s-hyroxvhesrl)rulionyl)alcobenzene (3). To a stirred, salt-ice cooled aqueous sulfuric acid (6 M, 10 mL) solution containing 4-aminophenyl6-hydroxyhexyl sulfone (2.0 g, 7.8 mmol) was added dropwise an aqueous sodium nitrite (0.6 g, 8.7 mmol) solution. The reaction temperature was maintained below 5 “C throughout the reaction to prevent the

Macromolecules, Vol. 26, No. 20, 1993 decompositionof the intermediate diazonium salt. After 15min of stirring, NJV-dimethylaniline (0.95 g, 7.8 mmol) was added, and the resulting red solution was stirred for additional 30 min. Sodium hydroxide (NaOH) (10 5% ) was slowly addedto neutralize the reaction mixture to pH = 6. The precipitate formed was fiitered out, washed extensively with water and cold ethanol, and vacuum dried. The orange solid (2.48 g, 82 % ) was further purified by recrystallization from ethanol. lH NMR (CDClS): 6 7.98 (m, br, 6H), 6.79 (d, J = 9.0 Hz, 2H), 3.61 (t,J = 6.2 Hz, 2H), 3.15 (m, br, 8H), 1.75-1.37 (m, br, 8H). Anal. Calcd for C a n N s O a S C, 61.70; H, 6.94; N, 10.80; S, 8.23. Found C, 61.82; H, 7.11; N, 10.76; S, 8.33. Monomer 1. Methacryloyl chloride (1.1g, 10.3 mmol) in 5 mL of dry THF was added dropwise to a stirred THF solution (15mL) of 3 (2.0 g, 5.1 mmol) and dry pyridine (0.8 g, 10.3 mmol). The reaction mixture was refluxed under argon for 14h and then evacuated to pump out THF. To the residue was added 1% NaOH aqueous solution, and the resulting mixture was extracted three times with chloroform. The chloroform layer was washed three times with brine and dried over anhydrous magnesium sulfate. After evaporating the solvent, the residue was purified by column chromatography (silica gel; chloroform/ethylacetate, 1:4). After vacuum drying, 1.75 g (75%)of red solid product was obtained. 1H NMR (CDCl3): 6 (see Figure 2 for the numbered structure) 7.98 (m, br, 6H, H12,19,16),6.78 (d, J = 9.2 Hz, 2H, H17), 6.07 (8, lH, Hid,), 5.54 (m, lH, HI*), 4.11 (t,J = 6.6 Hz, 2H, H6), 3.14-3.08 (m, br, 8H, H ~ o J ~1.93 ) , (8, 3H, Hs), 1.65-1.38 (m, br, 8H, Hm). "C NMR (CDCb): 6 167.3 (C4), 156.2 (Cu), 153.1 (cia),143.4 (cis), 138.1 ( C d , 136.3 (Cz), 129.0 ( C d , 125.8 ( C d , 64.4 (C6),56.2 ( C d , 42.2 ( c i g ) , 125.3 (Ci), 122.6 (Cis), 111.4 (ci~), 28.2 (Ce), 27.8 (ca),25.4 (c,),22.6 (cg), 18.2 (cs). And. Calcd for C&S~NSOS: C, 63.02; H, 6.78; N, 9.19; S, 7.00. Found: C, 62.68; H, 6.85; N, 8.92; S, 6.93. 4- [Bis (2- ((met hy It hio )met hoxy )et h y l)amino]-4'- ((6- hydroxyhexy1)sulfonyl)ambenzene (4). The diazonium salt of 4-aminopropyl6-hydroxyhexylsulfone was prepared using the same procedure as that for the preparation of 3. To this ice cooled diazonium salt solution was added sodium acetate tetrahydrate to adjust the pH to ca. 2. Compound 1 was then added with vigorous stirring, and the solution turned red immediately. The reaction mixture was adjusted to pH 6 by adding 10% NaOH and then extracted with dichloromethane. The organic layer was washed with brine, dried over magnesium sulfate, and concentrated. The resulting red oil was further purified by column chromatography (silica gel; ethyl acetate/ hexanes, 3:2). 1H NMR (CDCl3): 6 7.97 (m, br, 4H), 7.89 (d, J = 9.1 Hz, 2H), 6.83 (d, J = 9.2 Hz, 2H), 4.64 (s,4H), 3.76 (m, br, 8H), 3.59 (t, J = 6.3 Hz, 2H), 3.08 (t, J = 7.6 Hz, 2H), 2.11 (8, 6H), 1.52-1.37 (m, br, 8H). Anal. Calcd for C ~ H ~ N S O &C, : 54.84, H, 6.85; N, 7.38; S, 16.87. Found C, 54.12; H, 6.70; N, 7.14; S, 16.41. Compound 5. Methacryloylchloride (2.9 g, 28.1 "01) in 10 mLof dry THF was dropped into a stirred THF solution (70 mL) of dry consisting of 8.0 g (14.0 mmol) of 4 and 2.2 g (28.1 "01) pyridine. The reaction mixture was refluxed under argon for 14 h and then evacuated to pump out THF. The remaining viscous redliquid was dissolved in chloroform and washed with 1% NaOH and brine. The concentrated chloroform solution was loaded onto a column of silica gel and purified by chromatography (ethyl acetate/chloroform, 41), affording 6.4 g (71%) of extremely viscous red oil. 1H NMR (CDCls): 6 7.97 (m, br, 4H), 7.90 (d, J = 9.1 Hz, 2H), 6.83 (d, J = 9.2 Hz, 2H), 6.08 (8, lH), 5.54 (s, lH), 4.64 (s,4H), 4.11 (t,J = 6.6 Hz,2H), 3.76 (m, br, 8H), 3.12 (t, J = 7.9 Hz, 2H), 2.11 (a, 6H), 1.93 (8, 3H), 1.77-1.37 (m, br, 8H). Anal. Calcd for C&&O&: C, 56.48; H, 6.81; N, 6.59; S, 15.07. Found C, 56.19; H, 7.01; N, 6.13; S, 15.04. Monomer 2. A solution containing 5 (3.9 g, 6.7 mmol), silver nitrate (12.0 g, 70.5 mmol), 2,6-lutidine (4.6 g, 42.3 mmol), THF (32 mL), and water (8 mL) was vigorously stirred overnight at room temperature. THF was pumped out, and chloroform was added. The chloroform solution was washed five times with copper(I1) sulfate aqueous solution and then three times with brine. The chloroform solutionwas dried over magnesium sulfate and concentrated. The orange solid obtained was purified twice by column chromatography (silicagel; ethyl acetate/chloroform/ methanol, 91:1), yielding 1.9g (60%)of pure product. 1H NMR

Dipole Alignment of Poled Nonlinear Optical Polymers 6306 (CDCb): 6 (see Figure 1for the numbered structure of monomer 2) 7.96 (m, br, 4H, Hla,la), 7.89 (d,J = 8.0 Hz,2H, Hid, 6.82 (d, J = 8.8 Hz, 2H, H17), 6.07 (e, lH, HI&),5.54 (m, lH, H d , 4.10 (t,J = 6.5 Hz, 2H, Ha), 3.96 (t, J = 4.4 Hz, 4H, Ha), 3.77 (t, J 4.3 Hz, 4H, Hlg), 3.10 (t,J = 7.7 Hz,2H, Hie), 1.92 (8,3H, Hg), 1.76-1.61 (m, br, 4H, Hap), 1.39 (m, br, 4H, H,,& l8C NMR (CDCb): 6 167.5 (C4), 166.1 ((23, 153.0 (Cia), 143.6 (Ci& 138.2 (Cii), 136.3 (Cz), 129.0 (Ciz), 126.1 (Cis), 125.4 (Ci), 122.6 (Cis), 112.3 (CI~), 64.3 (cs),60.4 (ca),56.3 (cia), 55.2 (cis), 28.2 (Ce), 28.1 (CS), 25.5 (C,), 22.6 (Cd, 18.3 ((28). Anal. Calcd for C & ~ a O e S : C,60.31;H,6.77;N,8.12;5,6.19. Found c,60.13; H, 6.79; N, 8.04; S, 6.16. Polymer 1. Into a polymerizationtube was placed monomer 1(0.46g,1.0mmol),MMA(0.30g,3.0mmol),~N(3.7mg,0.02

mmol), and chloroform (4 mL). The tube was cooled in a dry ice bath, evacuated to 0.05 mmHg and then sealed. The polymerization solution was heated at 60 "C for 48 h, cooled to room temperature and then poured into 60 mL of vigorously stirred methanol. The resulting precipitate was fiitered out, washed with methanol, and dried. The polymer was further purified by dissolving in chloroform and reprecipitating in methanol. The product obtained was an orange powder (0.21 g, 27%). lH NMR (CDCb): 6 7.94 (m, br, 6H), 6.77 (d, J = 8.6 Hz, 2H), 3.89 (vb, 2H), 3.59 (vb, 12H), 3.13 (vb, 8H), 1.87-0.84 (m, vb). lSC NMR (CDCb): 6 178.0-177.0(m), 155.9,153.3,143.3,138.3,129.1,126.2,

122.6,111.7,64.7,56.2,54.2,52.4,44.8,44.5,40.4,27.9,27.7,25.6, 22.6, 18.8 (m), 16.4 (m). Polymer 2. Polymer 2 was synthesized using a similar procedure to that for the preparation of polymer 1. A sealed polymerizationtube containing 0.35 g (0.68 "01) of monomer 2,0.20 g (2.03 "01) of MMA, 4.5 mg (0.027 "01) of AIBN,and 3 mL of dimethyl sulfoxide (DMSO) was heated in a bath at 60 OC for 86 h. The resulting solution was poured into 50 mL of methanol with vigorous stirring to obtain a gel-like solid. The solid was dried in vacuum at 70 "C for 48 h, washed with chloroform, redissolved in DMSO, and then reprecipitated in methanol, yielding 0.25 g (45%) of red powder. lH NMR (DMSO-d6): 6 7.94 (m, br, 4H), 7.78 (d, J = 7.5 Hz, 2H), 6.86 (d, J = 7.4 Hz, 2H), 4.84 (br, 2H), 3.81 (vb, 2H), 3.58-3.49 (m, vb), 1.72-0.70(m,vb). 'Bc NMR (DMSO-&): 6 178.0-177.0 (m), 155.4, 15,20,142.4,138.4,129.0,125.7,122.2,111.5,58.1,54.6,53.3,51.7, 44.2, 43.9, 27.9, 27.0, 25.0, 22.2, 18.3 (m), 16.1 (m). Polymer 3. Polymer 3 was synthesized using the above procedure. A polymerization tube containing monomer 2 (0.35 g, 0.68 mmol), MMA (0.14 g, 13.5 mmol), HEMA (0.09 g, 0.68 mmol), AIBN (4.4 mg, 0.027 mmol), and DMSO (3 mL) was vacuum sealed and heated at 60 OC for 86 h. After cooling to room temperature, the resulting solution was poured into 100 mL of vigorously stirred methanol. A gel-like precipitate was formed, which was subsequently dissolved in DMSO and precipitated in methanol again. The precipitate was collected by filtration, washed with methanol and chloroform, and dried, affording 0.32 g (55%)of orange powder. lH NMR (DMSO-de): 6 7.99 (br, 4H), 7.78 (br, 2H), 6.86 (br, 2H), 4.85 (br, 2H, -OH), 4.78 (br, lH, -Om, 3.87 (vb), 3.58-3.48 (vb), 1.75-0.71 (vb). Cross-LinkedFilm. A dry THF solution containing polymer 2 and the cross-linker (4,4t-diisocyanato-3,3t-dimethoxybiphenyl) in a 1:1molar ratio was spin cast into a thin f i i on a glass slide substrate. The film was heated in a sealed oven at 160 OC under argon atmosphere to initiate cross-linking. FTIR was employed to monitor the reaction between the hydroxy and isocyanate groups. NLO Measurements. Polymer 1 is soluble in chloroform and polymer 2 and polymer 3 are soluble in THF. The polymer solutions, with or without the cross-linker, were fdtered through 0.2-pm syringe filters and then spin cast onto indium-tin oxide (ITO) coated glass slides. The f i b s were vacuum dried and stored in a drybox before electric poling and NLO measurement. An ITO-grounded corona-discharge setup, with a tipto-plane distance of 2.0 cm, was used to pole the f i i at elevated temperatures. The poling conditions are as follows: (1) for polymer 110 kV, 125 OC, and 45 min; (2) for polymer 2 with the cross-linker 5 kV, 140-160 OC, and 2 h; (3)for polymer 3 with the cross-linker 5 kV, 150 OC and 2 h. The film thickness of 0.61.5 pm was measured by a Dektak IIA profiier.

Macromolecules, Vol. 26, No.20, 1993

5306 Xu et al.

The absorption spectra of the polymer f i i , before and after electric fided poling, were measured with a Varian Cary 2415 spectrophotometer operating in the wavelength range 300-2500 nm. A Spectro-PhyeicsDCR-11modelocked, Q-switchedNdYAG laser (X = 1.064 pm) with a pulse width of